The present invention relates to an improvement in a semiconductor strain gauge and, more particularly, to an improvement in a semiconductor strain gauge wherein a semiconductor substrate on which a strain gauge stripe is formed is bonded to an elastic load plate.
It is well known that a semiconductor strain gauge utilizing a piezoresistive effect of a semiconductor has output sensitivity several ten times as high as that of a conventional wire strain gauge. A known semiconductor strain gauge employs a semiconductor substrate having a strain gauge stripe formed on one major surface thereof by diffusion or epitaxial growth. A semiconductor strain gauge obtained by bonding such a semiconductor substrate to an elastic metal load is called a load plate type semiconductor strain gauge. This type of semiconductor strain gauge is described in, for example, the specification of U.S. Pat. No. 3,662,234 granted to Ishii on May 9, 1972.
A load plate type semiconductor strain gauge is advantageous in that, a load plate, even when it consists of a metal diaphragm or casing, permits a semiconductor substrate to be bonded thereto comparatively easily. However, the output of this type of semiconductor gauge is affected by a difference between the coefficient of thermal expansion of a semiconductor substrate and that of a metal load plate, the type of bonding agent inserted between the semiconductor substrate and metal load plate, and the bonding conditions. This causes a zero point drift and hysteresis in the output. Thus, the above-mentioned type of semiconductor strain gauge is not satisfactory with respect to its sensitivity, precision and reliability.